1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) and, more particularly, to its control during an engine restart event.
2. Description of the Prior Art
A HEV is a vehicle configured with a hybrid propulsion system that utilizes at least two different sources of torque for propelling the vehicle. As one non-limiting example, a hybrid propulsion system may combine a conventional propulsion system that includes an internal combustion engine and a stepped-ratio change automatic transmission with an electric propulsion system that includes one or more electric motors and a rechargeable energy storage device, such as an electric battery, that can power the electric motors or store energy to improve fuel economy over the conventional vehicle. A hybrid electric vehicle typically provides different powertrain operating modes with the engine running or shutdown depending on the vehicle operating conditions, battery conditions, and driver's propulsion request. Hence, one of the major functions that an HEV provides is the ability to start or stop the engine during certain conditions. When the engine is running, the electric portion of the propulsion system may also be used to assist the engine in providing the required vehicle propulsion. During the conditions when the engine is shutdown, the driver's propulsion request can be provided entirely by the electric motor.
Motor vehicles can be designed to employ certain aspects of hybrid electric technology to reduce fuel consumption, but without use of a hybrid drivetrain. In such vehicles, called micro-HEVs, shutting down the engine during conditions where the engine operates at idle speed will be used to reduce fuel consumption and reduce emissions in a conventional powertrain that includes an internal combustion engine and a step-change automatic transmission, but no electric machine for driving the wheels. The primary condition that is checked by the micro-HEV powertrain control system before stopping the engine is that the driver has applied the brakes and the vehicle is stopped since the engine would typically be idling during these conditions in a conventional vehicle. Once the driver releases the brake pedal indicating a request for vehicle propulsion, the powertrain control system will automatically restart the engine. In a micro-hybrid electric powertrain having an automatic transmission, coordinated transmission control before, during and after an engine start is critical to acceptable vehicle performance. Specifically, control of clutch torque capacity during the engine start directly controls the amount of engine torque transferred to the wheels for vehicle propulsion. Moreover, any delays in clutch torque capacity application directly result in non-responsive vehicle propulsion and poor vehicle performance. Furthermore, in vehicle applications using a dual-clutch automatic transmission without a torque converter, often called a powershift transmission, any excessive and non-coordinated clutch torque capacity control during an engine start can potentially lead to engine stall and/or rough creep and launch performance.
One method commonly applied to address these issues simply keeps the transmission fully in gear whenever the gear range selector lever, i.e., the gear lever, is in a drive position, which may include DRIVE, REVERSE, MANUAL range positions. This technique, often referred to as Start-In-Gear (SIG), provides minimal transmission control coordination before and during the engine start since the transmission is to remain engaged in gear. Hence delayed vehicle propulsion is not be perceived by the driver.
There are deficiencies with the SIG approach such as excessive driveline excitation due to engine-start transients being transmitted to wheels since the transmission is fully engaged in gear. Furthermore, the SIG technique causes engine-start loading, since the transmission is fully engaged with its torque converter unlocked thus providing a torque flow path to the wheels. In addition, the SIG approach requires excessive electrical energy consumption, since an electric auxiliary pump must be fully activated to maintain sufficient hydraulic line pressure to keep transmission fully in gear before and during engine starts and stops for electro-hydraulically operated automatic transmissions. This results in excessive energy consumption while the engine is off.
The strategy is not applicable to dual clutch powershift automatic transmission applications without a torque converter. Finally, there is minimal or no transmission control coordination with the vehicle system and engine control under when SIG is employed.
A powertrain control system for a micro-HEV powertrain must provide an immediate response to a driver's request for vehicle propulsion when the engine is restarted. A strategy is needed to coordinate transmission and engine control during an engine start event while minimizing energy consumption in order to provide responsive, smooth, consistent and predictable vehicle propulsion performance.